Conventionally, in view of global environmental problems due to exhaust gases or the like, an improvement in fuel consumption has been pursued by lightening the body weight of transportation machines such as automobiles. Therefore, in particular, aluminum alloy forgings composed of AA or 6000 series aluminum alloy (Al—Mg—Si series) according to the JIS standard, and the like are used for structural materials or structural parts of transportation machines such as automobiles, and particularly for underbody parts such as upper arms and lower arms. The 6000 series aluminum alloy forgings have high strength, high toughness, and relatively excellent resistance to corrosion. The 6000 series aluminum alloys also have excellent recyclability because of a small number of alloy elements, and easy reusability of scraps as 6000 series aluminum alloy molten raw materials.
Each of the 6000 series aluminum alloy forgings is produced by performing hot forging (die forging) such as mechanical forging or hydraulic forging after homogenizing heat treatment of an aluminum alloy cast material, and then performing so-called tempering treatment including solution, quenching treatment, and artificial aging treatment. As a raw material for forging, besides the cast material mentioned above, an extruded material obtained by extruding a cast material once may also be used.
To improve the strength and toughness of the aluminum alloy forgings, various attempts have been made to improve the microstructures of the forgings. For example, it has been proposed in Patent Documents 1 and 2 that the average grain size of the crystal precipitates (crystallized substances or precipitates) of a 6000 series aluminum alloy forging is decreased to 8 μm or below, and a dendrite secondary arm spacing (DAS) is decreased to 40 μm or below to further increase the strength of the aluminum alloy forging.
In Patent Documents 3 to 5, it has been proposed to further increase the strength and toughness of an aluminum alloy forging by controlling the average grain size, average spacing, or the like of crystal precipitates in the crystal grains of a 6000 series Al alloy forging or on the grain boundaries thereof. The control can increase corrosion resistance even to intergranular corrosion, stress corrosion cracking, or the like. In addition to the control of these crystal precipitates, it has also been proposed to add a transition element having the effect of refining crystal grains, such as Mn, Zr, or Cr, to refine the crystal grains or change them into subcrystal grains, and improve fracture toughness and fatigue properties.
However, the 6000 series Al alloy forging has the tendency to produce coarse crystal grains by recrystallization of a worked structure in a forging step and a solution treatment step. When the coarse crystal grains are produced, higher strength and higher toughness cannot be achieved even by controlling the microstructure, and resistance to corrosion is also decreased. In addition, in these documents, a work temperature in the forging is as relatively low as less than 450° C., and it has been actually difficult to refine the target crystal grains or change them into subcrystal grains by hot forging at such a low temperature.
On the other hand, it has been proposed in Patent Documents 6 to 10 that, in order to suppress the production of coarse crystal grains by recrystallization of the worked structure, a transition element having the effect of refining crystal grains, such as Mn, Zr, or Cr, is added, and then hot forging is started at a relatively high temperature of 450 to 570° C.    Patent Document 1: Japanese Unexamined Patent Application Publication No. Hei 07-145440    Patent Document 2: Japanese Unexamined Patent Application Publication No. Hei 06-256880    Patent Document 3: Japanese Patent Publication No. 3684313    Patent Document 4: Japanese Unexamined Patent Application Publication No. 2001-107168    Patent Document 5: Japanese Unexamined Patent Application Publication No. 2002-294382    Patent Document 6: Japanese Unexamined Patent Application Publication No. Hei 05-247574    Patent Document 7: Japanese Unexamined Patent Application Publication No. 2002-348630    Patent Document 8: Japanese Unexamined Patent Application Publication No. 2004-43907    Patent Document 9: Japanese Unexamined Patent Application Publication No. 2004-292937    Patent Document 10: Japanese Unexamined Patent Application Publication No. 2004-292892